Attachments: Drag chains

Drag chains are attachments to a line which model straight chains that hang down from the line. They apply weight, buoyancy and drag forces to the node to which they are attached, but not any added mass effects, as described in detail under drag chain theory.

Drag chains include two facilities that can be important in modelling towed systems. Firstly, the chain's drag coefficients can vary with the incidence angle of the relative flow; this enables modelling the effect that as the relative flow increases the chain hangs at a greater angle to the vertical and so fluid drag generates more lift, which is applied to the line. Secondly, drag chains interact with the seabed (in a simple manner); if the node comes closer to the seabed than the chain length, then the seabed provides a supporting reaction force and a friction force, both of which are applied to the node.

Each drag chain attachment is of a named drag chain type, from which it inherits all its properties. The drag chain types are specified on the attachment types form and have the following data.

Name

Used to refer to the drag chain type.

Length

Length of the drag chain.

Effective diameter

The effective diameter of the drag chain is the diameter of the cylinder which has the same displaced mass per unit length.

Mass

Mass per unit length. Mass is assumed to be uniformly distributed along the length of the drag chain.

Friction coefficient

Coefficient of friction for contact with the seabed. This coefficient is used for all directions of friction. The value can be set to '~', in which case the drag chain will instead use the axial friction coefficient of the node to which the drag chain is attached.

Drawing

Defines the pen used for drawing drag chains of this type.

Drag coefficients

The fluid drag forces on the chain are determined from a table of the normal and axial drag coefficients, as a function of the incidence angle $\alpha$ between the relative velocity vector and the drag chain. So $\alpha{=}0°$ means flow axially along the drag chain and $\alpha{=}90°$ means flow normal to the drag chain.

Coefficients are given for a range of incidence angles between 0° and 90°, and linear interpolation is used to obtain coefficients for intermediate angles. The graph button shows the resulting coefficient variation. Symmetry is used to obtain coefficients for angles outside the range 0° to 90°.